normal low-flow period between June and September, rose 3.6 feet 

 (Forbes and Richardson, 1919: 141 ). 



After approximately 1910, as the pollution load increased, cri- 

 tically low dissolved oxygen levels in the water and putrescent con- 

 ditions in the bottom muds occurred further and further downstream 

 with detrimental effect on food organisms and fish (Richardson, 1921b: 33-36, 75). 

 Richardson believed that in the 1915-1920 period the area in which the 

 bottom fauna was drastically reduced or obliterated was expanding down- 

 stream at the rate of 16 miles per year. By 1920, the bottom fauna in 

 the river and bottomland lakes as far downstream as Browning (mile 

 97.0) had been affected. Between 1923 and 1^25 there was a recovery 

 in the benthos in the middle reach of the Illinois River (Richardson, 

 1928: 401-402). 



Recent studies have shown that benthic populations in the middle 

 reach of the Illinois River have again been reduced since the 1920' s. 

 Sparks (1975: 53-54) and Anderson (1977: 47-54) have described the 

 die-off of fingernail clams in the middle reach of the Illinois River. 

 Anderson (1977: 47-48) noted that mayflies were absent from the 

 samples taken in the middle reach of the river. Benthic studies 

 conducted at three power plant sites on the middle and lower sections 

 of the Illinois River showed that the greatest diversity of organisms 

 was obtained in the lower reach at Meredosia (mile 70.8). 



Table 3 compares the average density of benthic organisms found 

 by Richardson in river border and side channel habitats with the 

 densities found in three more recent studies. Between 1915 and 1964 

 the number of midges and oligochaete worms increased dramatically, 

 perhaps indicating that the organic load in the river had increased, 

 while the average dissolved oxygen levels had slightly decreased. Be- 

 tween 1964 and the 1970 's, the oligochaete worms and midges declined to 

 intermediate levels, while the mayflies increased substantially — 

 these results probably indicate a decline in the organic load and an 

 increase in the average dissolved oxygen level. The snails and finger- 

 nail clams increased between 1915 and 1964, as did the leeches, some of 

 which prey on clams and snails. Fingernail clams generally thrive in 



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